For both sides, the harsh rhetoric may actually serve a useful purpose this year. The 2012 vote figures to be a close one in which energizing the base of each party is the top priority, because there are so few undecided voters up for grabs this year. Tough campaign talk tends to fire up a party’s core.

But for Mr. Obama, the tone could damage his political brand of optimism that had appeal across the political spectrum in 2008. For Mr. Romney, the negativity distracts from his message on the economy, which polls show as Mr. Obama’s biggest vulnerability.

Neither side shows any signs of curtailing the negativity. The Obama campaign is planning an onslaught of attacks based on the budget crafted by Wisconsin Rep. Paul Ryan, the Republican vice-presidential candidate. The Romney campaign is running tough ads that accuse Mr. Obama of letting welfare recipients off the hook on requirements that they seek work. Each side is bitterly protesting the other’s ads.

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One effect of such early negativity is that both candidates figure to be battered by November, and voters could become fatigued earlier. And that could reduce even further the number of swing voters participating on election day—and increase even further the importance of turning out each party’s base.

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The negative strategy for each campaign has become clearer in recent days.

The Obama campaign decided more than a year ago to define Mr. Romney early, on its terms, in an attempt to make the election at least in part a referendum on him and his business record rather than what most re-election campaigns tend to be, which is a referendum on the incumbent. The result has been a negative message designed to raise doubts about whether Mr. Romney is trustworthy and stump speeches where the president takes on his opponent by name.

Mr. Romney’s campaign is seeking to drive up negative views of Mr. Obama, who remains personally fairly popular even as ratings of his job performance have stagnated. The campaign, seeing a potential vulnerability for Mr. Obama on the issue, has recently launched ads criticizing Mr. Obama for not fulfilling his promise in 2008 to bring a more civil tone to politics. Mr. Romney repeatedly said Mr. Obama doesn’t understand America.

Over three days and a dozen stops across this battleground state this week, Obama offered a road map of how he will appeal to the moderate and independent voters who will help decide the 2012 election.

He talked about the usual subjects of taxes and preserving the government investments, such as education aid, that help the middle class. But he also cast himself as the one who better understands — and has actually lived — the plight of such voters.

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Ryan’s entrance into the race Saturday has emboldened Obama to intensify the contrast. Ryan is the “ideological leader” of the House Republicans, the president said — the author of a GOP budget proposal that would make deep cuts in federal programs and grant $5 trillion in tax breaks, much of which would go to wealthy Americans.

“He is an articulate spokesman for Governor Romney’s vision,” Obama said of Ryan in Dubuque. “I just happen to fundamentally disagree with his vision. My opponent and his friends in Congress, they believe that if you just get rid of more regulations on big corporations and big banks, and then you give more tax breaks to the wealthiest Americans, that that will automatically lead to jobs and prosperity for ordinary families. And I’m not exaggerating here, that’s their basic economic plan.”